Sustained Tumor Regression With Zenocutuzumab, a Bispecific Antibody Targeting Human Epidermal Growth Factor Receptor 2/Human Epidermal Growth Factor Receptor 3 Signaling, in NRG1 Fusion-Positive, Estrogen Receptor-Positive Breast Cancer After Progression on a Cyclin-Dependent Kinase 4/6 Inhibitor.